Anyone know how the lovecraftian stuff ties into Satanism? Or are the cult members not Satanists? I saw the dude Marty shot had the big pentagram tattoo
It doesn't tie into it at all. God/Satan doesn't exist in The Cthulhu Mythos, the only gods are the Great Old Ones (Cthulhu, Azathoth, Nyarlathotep, Yog-Sothoth, Hastur aka The King In Yellow, etc.)
Now, one of the Great Old Ones, Nyarlathotep aka The Crawling Chaos, is often mistaken for Satan and is worshiped as such. He appears as a handsome charismatic man that offers power and prestige to his followers, but it always leads to ruination and madness.
That's the thing about the Great Old Ones though, they're bad news. You've probably seen movies or read stories where someone sells their soul to the Devil in exchange for riches or power....the Great Old Ones have no interest in humans or souls. They want to wipe out all life on Earth, that's it. They don't make deals and they don't reward their followers. Only the most depraved, insane people (like Reggie Ledeux) would willingly worship them.
Side note:
Nic Pizzolatto is probably using Chambers "The King In Yellow" instead of directly referencing Lovecraft because "The King In Yellow" is public domain. No one holds the copy right to it so it's free. Also he gets the added bonus of it tying into The Cthulhu Mythos, anyway, for free. Lovecraft's stuff is kind of a legal nightmare as far as copyrights go.
Fred.